Heard flag formed part of vehicle (4)
I believe the answer is:
tyre
'part of vehicle' is the definition.
'tyre' can be an answer for 'part' (I have seen 'part of car' mean 'tyre' so perhaps 'part' could also mean 'tyre'). I'm not sure about the 'of vehicle' bit.
'heard flag' is the wordplay.
'heard' shows a homophone (sound like).
'flag' becomes 'tire' (both can mean to become weary).
'tire' sounds like 'TYRE'.
'formed' acts as a link.
Can you help me to learn more?
(Other definitions for tyre that I've seen before include "Old Phoenician port" , "Rubber covering on a wheel" , "Rubber wheel; port" , "Rubber wheel-covering" , "Rubber tube on a wheel" .)